Cytoplasmic tails of human complement receptor type 3 (CR3, CD11b/CD18) regulate ligand avidity and the...

Area of Science:

• Immunology
• Cell Biology
• Molecular Biology

Background:

• The human complement receptor type 3 (CR3, CD11b/CD18) plays a critical role in immune responses.
• Understanding the function of CR3's cytoplasmic tails is essential for elucidating its regulatory mechanisms.

Purpose of the Study:

• To investigate the role of CD11b and CD18 cytoplasmic tails in CR3 ligand binding and internalization.
• To determine how mutations in these tails affect CR3 function.

Main Methods:

• Generated truncation mutants of CD11b and CD18 in COS fibroblastoid cells.
• Assayed surface membrane expression, ligand binding (iC3b), and internalization of CR3.
• Utilized in vitro mutagenesis to create specific tail truncations and mutations.

Main Results:

• Truncation of CD11b (alpha 1119T) and CD18 (beta 728T) increased iC3b binding by 330% and 210%, respectively, indicating negative control of avidity by cytoplasmic tails.
• Internalization of wild type CR3 occurred via coated pits.
• Truncating the CD18 cytoplasmic tail, but not CD11b, significantly reduced coated pit-mediated internalization.
• A single F754A mutation in CD18's NPXF motif also impaired internalization.

Conclusions:

• Both CD11b and CD18 cytoplasmic tails negatively regulate CR3 receptor avidity.
• The CD18 cytoplasmic tail is crucial for CR3 internalization through coated pits, likely involving NPXF motifs.